Recent data support the role of post-transcriptional processing in the selection of mRNAs. We propose (1) to determine the extent to which post-transcriptional regulation functions in mammalian gene expression by comparing nuclear and cytoplasmic RNA complexity and sequence overlap between various adult rat tissues--in particular, to determine if histospecific mRNAs of one tissue are present in the nuclear RNA of another tissue; (2) to determine the extent of splicing and presence of modified bases at the splice junction in a general population of complex mRNAs from brain; and (3) clone neural-specific gene, S100, and compare processing intermediates in cells that make S100 mRNA and those in which S100 hnRNA sequences are turned over in the nucleus, with the eventual goal of finding potential tissue-specific processing enzymes.